Naval ships have in the past been used in attack modes, which is to say for striking at the ships, aircraft and terrain of hostile powers. In conjunction with Ballistic Missile Defense (BMD) efforts, ships are increasingly being adapted for defense of a territory against ballistic missiles by being outfitted with equipment for surveillance, ballistic missile target tracking, target discrimination, target typing, and target cueing. In many cases, ships are provided with anti-ballistic-missile capabilities, such as antimissiles for destroying missiles, including intercontinental missiles, which are a threat to friendly territory. Various ballistic missile detection, tracking, and antimissile guidance systems are described in U.S. patent application Ser. Nos. 11/356,675 filed Feb. 16, 2006 in the name of Denis et al., 11/430,535 filed May 9, 2006 in the name of Pedersen et al., 11/430,644 filed May 9, 2006 in the name of Pedersen, and 11/430,647 filed May 9, 2006 in the name of Pedersen et al.
Among the more recent improvements in defense against ballistic missiles is that of communicating missile tracking information among a plurality of sites, which sites may be land-based, ships at sea, aircraft and satellites. This communication allows a sensor(s) at a first site, as for example sensors at a picket ship near a hostile shore, to sense the launch of a hostile missile from the hostile territory. The picket ship may attempt to engage and destroy the hostile missile, but may be unable to because of the head start of the hostile missile before it is initially detected. By sending the picket ship's sensed data, or processed sensed data, to antimissile-equipped platforms farther out to sea, the antimissile defenses of these additional assets can be brought to bear against the hostile missile, with a probability of success which is increased by virtue of the advance or early knowledge of the location, heading and speed of the hostile missile.
When a single ship or other platform performs the detection of the hostile missile and also engages the missile with its antimissile defenses, all of the sensing can be done in a local or ship's reference frame. However, when sensed information from a picket ship is sent to other ships or platforms, there is a potential for error attributable to undesired differences between the local reference frames and a true reference frame or geodetic coordinates. These differences, if not taken into account, can result incorrect interpretation of the data at the remote or data-receiving site, which in turn may cause the antimissile launched by the remote site to initially be directed incorrectly. If the antimissile is initially launched in an incorrect direction, it may not be possible to correct the heading of the antimissile in time to destroy the hostile missile or target. The problem has been solved in the past by relative gridlock techniques. “Relative Gridlock” is known in the art and is described in, for example, R. E. Helmick, J. E. Conte, T. R. Rice, Absolute Alignment of Sensors, NSWCDD/TR-96/46, Dahlgren Div., Surface Warfare Center, and H. O. Ladd, An Optimal Gridlock Solution in Closed Form That can be Implemented Recursively, Lockheed Martin Technical Report, CS-Z-MIS-A-2141, June 1997. The relative gridlock technique is cumbersome.
In general, the attitude reference for a ship is provided by an inertial navigation system (INS), which attempts to provide attitude information or data in terms of local east-north-up (ENU) coordinates. Such inertial navigation systems are subject to drift and bias errors, so the data indicative of attitude may be incorrect on each intercommunicating ship. The INS is initially calibrated to the true North in-port by star sighting on Polaris on a clear night. This procedure is lengthy, and may take as much as 72 hours due to the need for correction of the gyros for the Earth's rotation, which is at a relatively slow rate. At sea, an accurate “local level” can be maintained by corrections available from bathometric data tables for compensation of the inertial gyro vertical deflection error attributable to uneven mass distribution at the sea bottom. “East” and “Up” of the inertial navigation system are not calibrated in port, but instead are calibrated in a laboratory.
A significant improvement in the coordinate transformation accuracy of the ship target data to the earth-centered-fixed (ECEF) coordinates can be achieved by reducing the attitude misalignment between the East-North-Up (ENU) reference established by the ship's inertial navigation unit and true geodetic ENU reference. At sea following the static calibration, recalibration of the inertial navigation system is not performed for lack of a suitable calibration method. Thus, the inertial navigation systems of each of the ships involved in a multiple-platform defense against hostile ballistic-type missiles tend to drift relative to each other, giving rise to unwanted errors which cumulatively may degrade antimissile defense.